Wind turbines are vulnerable to being struck by lightning; sometimes on the tower, nacelle and the rotor hub, but most commonly on the blades of the turbine. A lightning strike event has the potential to cause physical damage to the turbine blades and also electrical damage to the internal control systems of the wind turbine. Wind turbines are often installed in wide open spaces which makes lightning strikes a common occurrence. Accordingly, in recent years much effort has been made by wind turbine manufacturers to design wind turbines so that they are able to manage effectively the energy imparted to them during a lightning strike in order to avoid damage to the blade and the associated cost of turbine down-time during blade replacement.
Lightning protection systems for wind turbine blades are known. In one example, an electrically conductive lightning receptor element is arranged on an outer surface of the blade to receive a lighting strike. Since the receptor element is electrically conductive, lightning is more likely to attach to the receptor element in preference to the relatively non-conductive material of the blade. The receptor element is connected to a cable or ‘down conductor’ that extends inside the blade to the root and from there connects via an armature arrangement to a charge transfer route in the hub, nacelle and tower to a ground potential. Such a lightning protection system therefore allows lightning to be channelled from the blade to a ground potential safely, thereby minimising the risk of damage. However, the discrete receptor elements are relatively complex to install during fabrication of the blade and, moreover, they leave a significant portion of blade area exposed to a risk of lightning strike.
Observation of the effects of lightning strikes on turbine blades has revealed that the highest proportion of lightning strikes happen at the blade tips. To address this, WO2005/031158 proposes a turbine blade having a solid metal tip. Although the solid metal tip provides a robust conductive body to withstand a high number of lightning strikes, in some circumstances lightning may still strike the blade in-board of the tip thereby having the potential to cause blade damage.
A further lightning protection is described in WO2013/007267, which proposes demarcating a turbine blade into a plurality of different zones, each zone being provided with a different protective measure that is selected depending on the expected impact of a lightning strike within that blade zone. Here, a high risk strike zone such as the tip can be protected with robust protection measures such as a solid metal tip, whilst protective measures in low risk strike zones can potentially be removed altogether.
It is against this context that the invention has been devised.